Internet Engineering Task Force Lehtovirta, Naslund, Norrman
(Ericsson)
INTERNET-DRAFT
EXPIRES: March 2007 October 2006
Integrity Transform Carrying Roll-over Counter<draft-lehtovirta-srtp-rcc-06.txt>
Status of this memo
By submitting this Internet-Draft, each author represents that any
applicable patent or other IPR claims of which he or she is aware
have been or will be disclosed, and any of which he or she becomes
aware will be disclosed, in accordance with Section 6 of BCP 79.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF), its areas, and its working groups. Note that
other groups may also distribute working documents as Internet-
Drafts.
Internet-Drafts are draft documents valid for a maximum of six
months and may be updated, replaced, or obsoleted by other documents
at any time. It is inappropriate to use Internet-Drafts as
reference material or to cite them other than as "work in progress".
The list of current Internet-Drafts can be accessed at
http://www.ietf.org/ietf/1id-abstracts.txt.
The list of Internet-Draft Shadow Directories can be accessed at
http://www.ietf.org/shadow.html.
This Internet-Draft will expire in March 2007.
Abstract
This document defines an integrity transform for SRTP [RFC3711],
which allows the roll-over counter (ROC) to be transmitted in SRTP
packets as part of the authentication tag. The need for sending the
ROC in SRTP packets arises in situations where the receiver joins an
ongoing SRTP session, and needs to quickly and robustly synchronize.
The mechanism also enhances SRTP operation in cases where there is a
risk of loosing sender-receiver synchronization.

INTERNET-DRAFT srtp-rcc October, 2006
TABLE OF CONTENTS
1. Introduction...................................................22. The transform..................................................43. Transform modes................................................54. Parameter negotiation..........................................65. Security Considerations........................................86. IANA Considerations...........................................107. Acknowledgements..............................................108. Author's Addresses............................................119. References....................................................111. Introduction
When a receiver joins an ongoing SRTP session, out of band signaling
must provide the receiver with the value of the ROC the sender is
currently using. For instance, it can be transferred in the Common
Header Payload of a MIKEY [RFC3830] message. In some cases the
receiver will not be able to synchronize his ROC with the one used
by the sender even if it is signaled to him out of band. Examples
of where synchronization failure will appear are:
1. The receiver receives the ROC in a MIKEY message together with
a key required for a particular continuous service. He does,
however, not join the service until after a few hours, at which
point the sender's sequence number (SEQ) has wrapped around, and
the sender hence has meanwhile increased the value of ROC. When
the user joins the service he grabs the SEQ from the first seen
SRTP packet and prepends the ROC to build the index. If
integrity protection is used, the packet will be discarded. If
there is no integrity protection, the packet may (if key
derivation rate is non-zero) be decrypted using the wrong session
key as ROC is used as input in session key derivation. In either
case, the receiver will not have its ROC synchronized with the
sender, and it is not possible to recover without out-of-band
signalling.
2. If the receiver leaves the session (due to being out of radio
coverage or because of a user action), and does not start
receiving traffic from the service again until after 2^15 packets
has been sent, the receiver will be out of synchronization (for
the same reasons as in example 1).
3. The receiver joins a service when the SEQ is close after
wraparound, say SEQ = 0x0001. The sender generates a MIKEY
message, and includes the current value of ROC, say ROC = 1, in
Lehtovirta et al. [Page 2]

INTERNET-DRAFT srtp-rcc October, 2006
the MIKEY message. The MIKEY message reaches the receiver, who
reads the ROC value and initializes its local ROC to 1. Now, if
a SRTP packet prior to wraparound, i.e., with a SEQ lower than 0,
say SEQ = 0xffff, was delayed and reaches the receiver as the
first SRTP packet he sees, the receiver will initialize its
highest received sequence number, s_l, to 0xffff. Next the
receiver will receive SRTP packets with sequence numbers larger
than zero, and will deduce that the SEQ has wrapped. Hence, the
receiver will incorrectly update the ROC and will be out of
synchronization.
4. Similarly to (3), since the initial SEQ is selected at random by
the sender, it may happen to be selected as a value very close to
0xffff. In this case, should the first few packets be lost, the
receiver may similarly end up out of synchronization.
These problems have been recognized in, e.g., 3GPP2 and 3GPP, where
SRTP is used for streaming media protection in their respective
multicast/broadcast solutions [BCMCS][MBMS]. Problem 4 actually
exists inherently due to the way SEQ initialization is done in RTP.
One possible approach to address the issue could be to carry the ROC
in the MKI field of each SRTP packet. This has the advantage that
the receiver immediately knows the entire index for a packet.
Unfortunately, the MKI has no semantics in RFC 3711 (other than
specifying master key), and a regular RFC 3711 compliant
implementation would not be able to make use of the information
carried in the MKI. Furthermore, the MKI field is not integrity
protected, and hence care must be taken to avoid obvious attacks
against the synchronization.
In this document a solution is presented where the ROC is carried in
the authentication tag of a special integrity transform in selected
SRTP packets.
The benefit of this approach is that the functionality of fast and
robust synchronization can be achieved as a separate integrity
transform, using the hooks existing in SRTP. Furthermore, when the
ROC is transmitted to the receiver it needs to be integrity
protected, to avoid persistent DoS attacks or transmission errors
bringing the receiver out of synchronization. (A DoS attack is
regarded as persistent if it can last after the attacker has left
the area, e.g., in this particular case an attacker could modify the
ROC in one packet and the victim would be out of synchronization
until the next ROC is transmitted). The above discussion leads to
that it makes sense to carry the ROC inside the authentication tag
of an integrity transform.
Lehtovirta et al. [Page 3]

INTERNET-DRAFT srtp-rcc October, 2006
1.1 Terminology
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in RFC 2119 [RFC2119].
2. The transform
The transform, hereafter called Roll-over Counter Carrying Transform
(or RCC for short), works as follows.
The sender processes the RTP packet according to RFC 3711. When
applying the message integrity transform, the sender checks if the
SEQ is equal to 0 modulo some non-zero integer constant R. If that
is the case, the sender computes the MAC in the same way as is done
when using the default integrity transform (i.e., HMAC-
SHA1(auth_key, Authenticated_portion || ROC)). Next the sender
truncates the MAC by 32 bits to generate MAC_tr, i.e., MAC_tr is the
tag_length - 32 most significant bits of the MAC. Next the sender
constructs the tag as TAG = ROC_sender || MAC_tr, where ROC_sender
is the value of his local ROC, and appends the tag to the packet.
See the security considerations section for discussions on the
effects of shortening the MAC. In particular note that a tag-length
of 32 bits gives no security at all.
If the SEQ is not equal to 0 mod R, the sender just proceeds to
process the packet according to RFC 3711 without performing the
actions in the previous paragraph.
The value R is the rate at which the ROC is included in the SRTP
packets. Since the ROC consumes four octets, this gives the
possibility to use it sparsely.
When the receiver receives an SRTP packet, it processes the packet
according to RFC 3711 except that during authentication processing
ROC_local is replaced by ROC_sender (retrieved from the packet).
This works as follows. In the step where integrity protection is to
be verified, if the SEQ is equal to 0 modulo R, the receiver
extracts ROC_sender from the TAG and verifies the MAC computed (in
the same way as if the default integrity transform was used) over
the authenticated portion of the packet (as defined in [RFC3711])
but concatenated with ROC_sender instead of concatenated with the
local_ROC. The receiver generates MAC_tr for the MAC verification
in the same was as the sender did. Note that the session key used
in the MAC calculation is dependent on the ROC, and during the
derivation of the session integrity key, the ROC found in the packet
Lehtovirta et al. [Page 4]

INTERNET-DRAFT srtp-rcc October, 2006
under consideration MUST be used. If the verification is
successful, the receiver sets his local ROC equal to the ROC carried
in the packet. If the MAC does not verify, the packet MUST be
dropped. The rationale for using the ROC from the packet in the MAC
calculation is that if the receiver has an incorrect ROC value, MAC
verification will fail, and the receiver will not correct his ROC
because of this.
If the SEQ is not equal to 0 mod R, the receiver just proceeds to
process the packet according to RFC 3711 without performing the
actions in the previous paragraph.
Since SRTCP already carries the entire index in-band, there is no
reason to apply this transform to SRTCP. Hence, the transform SHALL
only be applied to SRTP, and SHALL NOT be used with SRTCP.
3. Transform modes
The above given transform only provides integrity protection for the
packets that carry the ROC (this will be referred to mode 1). In
the cases where there is a need to integrity protect all the
packets, the packets that do not have SEQ equal to 0 mod R, MUST be
protected using the default integrity transform (this will be
referred to as mode 2).
Under some circumstances, it may be acceptable to not use integrity
protection on any of the packets; this will be referred to as mode
3. Without integrity protection of the packets carrying the ROC, a
DoS attack, that will prevail until the next correctly received ROC,
is possible. It should be made sure to carefully read the security
considerations in Section 5 before using mode 3.
In case no integrity protection is offered, i.e., mode 3, the
following applies. The receiver's SRTP layer SHOULD ignore the ROC
value from the packet if the application layer can indicate to it
that the local ROC is synchronized with the sender (the packet would
hence be processed using the local ROC). Note that the received ROC
still MUST be removed from the packet before continued processing.
In this scenario, the application layer feedback to the SRTP layer
need not be on a per-packet basis, and it can consist merely of a
boolean value set by the application layer and read by the SRTP
layer.
Thus, note the following difference. Using mode 2 will integrity
protect all RTP packets, but only add ROC to those having SEQ
divisible by R. Using mode 1 and setting R equal to one, will also
integrity protect all packets, but will in addition add ROC to each
Lehtovirta et al. [Page 5]

INTERNET-DRAFT srtp-rcc October, 2006
packet. Modes 1 and 2 MUST compute the MAC in the same way as the
pre-defined authentication transform for SRTP, i.e. HMAC-SHA1.
To comply with this specification, mode 1, mode 2 and mode 3 are
MANDATORY to implement. However, it is up to local policy to decide
which mode(s) are allowed to be used.
4. Parameter negotiation
RCC requires that a few parameters are signaled out of band. The
parameters that must be in place before the transform can be used
are integrity transform mode and the rate, R, at which the ROC will
be transmitted. This can be done using, e.g., MIKEY [RFC3830].
To perform the parameter negotiation using MIKEY, there is a need to
register three integrity transforms, RCCm1, RCCm2 and RCCm3 in Table
6.10.1.c of [RFC3830] for the three modes defined.
Table 1. Integrity transforms
SRTP auth alg | Value
--------------+------
RCCm1 | 2
RCCm2 | 3
RCCm3 | 4
Furthermore, the parameter R, must be registered in Table 6.10.1.a
of [RFC3830].
Table 2. Integrity transform parameter
Type | Meaning | Possible values
-----+-----------------------------+----------------
13 | ROC transmission rate | 16-bit integer
The ROC transmission rate, R, is given with the leftmost bit being
the most significant. R MUST be a non-zero unsigned integer. If
the ROC transmission rate is not included in the negotiation, the
default value of 1 SHALL be used.
To be able to use different integrity transforms for SRTP and SRTCP,
which is needed in connection to the use of RCC, the following
Lehtovirta et al. [Page 6]

INTERNET-DRAFT srtp-rcc October, 2006
additional parameters must be registered in Table 6.10.1.a of
[RFC3830]:
Table 3. Integrity parameters
Type | Meaning | Possible values
-----+-----------------------------+----------------
14 | SRTP Auth. algorithm | see below
15 | SRTCP Auth. algorithm | see below
16 | SRTP Session Auth. key len | see below
17 | SRTCP Session Auth. key len | see below
18 | SRTP Authentication tag len | see below
19 | SRTCP Authentication tag len| see below
The possible values for authentication algorithms (type 14 and 15)
are the same as for the "Authentication algorithm" parameter (type
2) in Table 6.10.1.a of RFC3830 with the addition of the values
found in Table 1 above.
The possible values for session authentication key lengths (type 16
and 17) are the same as for the "Session Auth. key length" parameter
(type 3) in Table 6.10.1.a of RFC3830.
The possible values for authentication tag lengths (type 18 and 19)
are the same as for the "Authentication tag length" parameter (type
11) in Table 6.10.1.a of RFC3830 with the addition that the length
of ROC MUST be included in the "Authentication tag length"
parameter. This means that the minimum tag length when using RCC is
32 bits.
To avoid ambiguities when introducing these new parameters that have
overlapping functionality to existing parameters in Table 6.10.1.a
of RFC3830, the following approach MUST be taken: If any of the
parameter types 14-19 (specifying behavior specific to SRTP or
SRTCP) and a corresponding general parameter (type 2, 3, or 11) are
both present in the policy, the more specific parameter SHALL have
precedence. For example, if the "Authentication algorithm" parameter
(type 2) is set to HMAC-SHA-1 and the "SRTP Auth. Algorithm" (type
14) is set to RCCm1, SRTP will use the RCCm1 algorithm, but since
there is no specific algorithm chosen for SRTCP, the more generally
specified one (HMAC-SHA-1) is used.
Lehtovirta et al. [Page 7]

INTERNET-DRAFT srtp-rcc October, 20065. Security Considerations
An analogous method already exists in SRTCP (the SRTCP index is
carried in each packet under integrity protection) and to the best
of our knowledge, the only security consideration introduced here is
that the entire SRTP index (ROC || SEQ) will become public since it
is transferred without encryption. (In normal SRTP operation, only
the SEQ-part of the index is disclosed). However, RFC 3711 does not
identify a need for encrypting the SRTP index.
It is important to realize that only every Rth packet is integrity
protected in mode 1, so unless R = 1, the mechanism should be seen
for what it is: a way to improve sender-receiver synchronization,
and not a replacement for integrity protection.
The use of mode 3 (NULL-MAC) introduces a vulnerability not present
in RFC 3711, namely, if an attacker modifies the ROC, the
modification will go undetected by the receiver, and the receiver
will lose cryptographic synchronization until the next correct ROC
is received. This implies that an attacker can perform a DoS attack
by only modifying every Rth packet. Because of this, NULL-MAC MUST
only be used after proper risk assessment of the underlying network.
Besides the considerations in Section 9.5 and 9.5.1 of RFC 3711,
additional requirements of the underlying transport network must be
met.
. The transport network must only consist of trusted domains. That
means that everyone on the path from the source to the destination
is trusted not to modify or inject packets.
. The transport network must be protected from packet injection,
i.e., it must be ensured that the only packets present on the path
from the source to the destination(s) originates from trusted
sources.
. If the packets, on their way from the source to the
destination(s), travel outside of a trusted domain, their
integrity must be assured (e.g., by using a VPN connection or a
trusted leased line).
In the (assumed common) case that the last link to the
destination(s) is a wireless link, the possibility that an attacker
injects forged packets here must be carefully considered before
using NULL-MAC. Especially, if used in a broadcast setting, many
destinations would be affected by the attack. However, unless R is
big, this DoS attack would be similar in effect to radio jamming,
which would be easier to perform.
Lehtovirta et al. [Page 8]

INTERNET-DRAFT srtp-rcc October, 2006
It must also be noted that if the ROC is modified by an attacker and
no integrity protection is used, the output of the decryption will
not be useful to the upper layers, and these must be able to cope
with the randomly looking data. In the case integrity protection is
used on the packets containing the ROC and the ROC is modified by an
attacker (and the receiver already has an approximation of the ROC,
e.g., by getting it previously), the packet will be discarded and
the receiver will not be able to decrypt correctly. Note however
that the situation is better in the later case, since the receiver
now can try different ROC values in a neighborhood around the
approximate value he already has.
As RCC is expected to be used in a broadcast setting where group
membership will be based on access to a symmetric group key, it is
important to point out the following. With symmetric key based
integrity protection, it may be as easy, if not easier, to get
access to the integrity key (often a combination of a low-cost
activity of purchasing a subscription and breaking the security of a
terminal to extract the integrity key) as being able to transmit.
A word of warning is in place when it comes to the choice of length
of the authentication tag. It shall be noted that, in contrast to
common MAC tags, there is a clear distinction made between the RCC
authentication tag and the RCC MAC. The tag is the container
holding the MAC (and for some packets also the ROC), and the MAC is
the output from the MAC-algorithm (i.e., HMAC-SHA1). The length of
the authentication tag with the RCC transform includes the four
octet ROC in some packets. This means that for a tag-length of n
octets, there is only room for a MAC of length n - 4, i.e., a tag-
length of n octets does not provide a full n-octet integrity
protection on all packets. There are five cases:
1. RCCm1 is used and tag-length is n. For those packets that SEQ
= 0 mod R, the ROC is carried in the tag and occupies four
octets. This leaves n - 4 octets for the MAC.
2. RCCm1 is used and tag-length is n. For those packets that SEQ
!= 0 mod R, there is no ROC carried in the tag. For RCCm1
there is no MAC on packets not carrying the ROC, so neither the
length of the MAC nor the length of the tag has any relevance.
3. RCCm2 is used and tag-length is n. For those packets that SEQ
= 0 mod R, the ROC is carried in the tag and occupies four
octets. This leaves n - 4 octets for the MAC (this is
equivalent to case 1).
Lehtovirta et al. [Page 9]

INTERNET-DRAFT srtp-rcc October, 2006
4. RCCm2 is used and tag-length is n. For those packets that SEQ
!= 0 mod R, there is no ROC carried in the tag. This leaves n
octets for the MAC.
5. RCCm3 is used. RCCm3 does not use any MAC, but the ROC still
occupies four octets in the tag for packets with SEQ = 0 mod R,
so the tag-length MUST be set to four. For packets with SEQ !=
0 mod R, neither the length of the MAC nor the length of the
tag has any relevance.
The conclusion is that in cases 1 and 3, the length of the MAC is
shorter than the length of the authentication tag. To achieve the
same (or less) MAC forgery success probability on all packets when
using RCCm1 or RCCm2, as with the default integrity transform in
RFC3711, the tag-length must be set to 14 octets, which means that
the length of MAC_tr is 10 octets.
It is recommended to set the tag-length to 14 octets when RCCm1 or
RCCm2 is used, and the tag-length MUST be set to four octets when
RCCm3 is used.
6. IANA Considerations
Please add the following to the IANA registry at
http://www.iana.org/assignments/mikey-payloads (This paragraph to be
removed after IANA processing).
According to Section 10 of RFC 3830, IETF consensus is required to
register values in the range 0-240 in the SRTP auth alg namespace
and the SRTP Type namespace.
It is requested to register the value 2 for RCCm1,the value 3 for
RCCm2 and the value 4 for RCCm3 in the SRTP auth alg namespace as
specified in Table 1 in Section 4.
It is also requested to register the value 13 for ROC transmission
rate in the SRTP Type namespace as specified in Table 2 in Section4.
It is also requested to register the values 14 to 19 according to
Table 3 in Section 4 to the SRTP Type namespace.
7. Acknowledgements
We would like to thank Nigel Dallard, Lakshminath Dondeti and David
McGrew for fruitful comments and discussions.
Lehtovirta et al. [Page 10]

INTERNET-DRAFT srtp-rcc October, 2006
Intellectual Property
The IETF takes no position regarding the validity or scope of any
Intellectual Property Rights or other rights that might be claimed
to pertain to the implementation or use of the technology described
in this document or the extent to which any license under such
rights might or might not be available; nor does it represent that
it has made any independent effort to identify any such rights.
Information on the procedures with respect to rights in RFC
documents can be found in BCP 78 and BCP 79.
Copies of IPR disclosures made to the IETF Secretariat and any
assurances of licenses to be made available, or the result of an
attempt made to obtain a general license or permission for the use
of such proprietary rights by implementers or users of this
specification can be obtained from the IETF on-line IPR repository
at http://www.ietf.org/ipr.
The IETF invites any interested party to bring to its attention any
copyrights, patents or patent applications, or other proprietary
rights that may cover technology that may be required to implement
this standard. Please address the information to the IETF at ietf-
ipr@ietf.org.
Copyright Notice
Copyright (C) The Internet Society (2006). This document is subject
to the rights, licenses and restrictions contained in BCP 78, and
except as set forth therein, the authors retain all their rights.
Disclaimer of Validity
This document and the information contained herein are provided on
an "AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE
REPRESENTS OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY AND THE
INTERNET ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES, EXPRESS OR
IMPLIED, INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF
THE INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED
WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.
Lehtovirta et al. [Page 12]